Direct coupled amplifier



Aug. 22, 1933. w. BURSTYN r AL 1,923,254

DIRECT COUPLED AMPLIFIER Filed June 22, 1932 INVENTORS Walther Bursiynand BY Walter TCLUSI'.%

ATTORNEY Patented Aug. 22, 1933 UNITED STATES DIRECT COUPLED AMPLIFIERWalther Burstyn and Walter Tausig, Berlin,

Germany, assignors to Radio Patents Corporation, New York, N. Y., aCorporation of New York Application June 22, 1932, Serial No. 618,707,and in Germany July 29, 1931 12 Claims.

Our invention relates to electronic valve systems and more particularlyto electronic cascade amplifiers in which a direct or conductivecoupling from the output electrode of one valve to the input electrodeof a subsequent valve is provided.

This type of amplifier is known as direct coupled amplifier orcontinuous current amplifier due to its use for amplifying very slowcurrent variations for relay operation and the like. It has the furtheradvantage of possessing a fiat top frequency response curve comprisingan extended band of frequencies, which makes its application especiallysuited for purposes where increased fidelity of reproduction is ofprimary importance such as for the amplification of sound and speechcurrents in public address systems, talking moving picture systems, andthe like.

Direct coupled amplifiers are especially suited for amplifyingtelevision signals dueto the extended signal frequency bands encounteredin television transmission.

It is an object of our invention to provide a direct coupled electronicvalve circuit of simple design and of increased efliciency anddependability in operation.

In direct coupled amplifier systems hereinbefore used and described inthe prior art, an increased operating voltage is usually requireddependent on the number of valves connected in cascade in case it isdesired to use a common voltage supply source for operating all of theamplifying valves.

Accordingly, it is another object of our invention to provide a circuitrequiring a decreased operating voltage compared with systems used inthe prior art.

A more specific object of our invention is to provide a direct coupledcascade system in which a self inductance coil may be used as a couplingmeans resulting in a simplified circuit arrangement and ease ofoperation.

These and further objects and aspects of our invention will become moreapparent from the followingdetailed description taken with reference tothe accompanying drawing, illustrating a few circuit arrangements inwhich the invention has been embodied. We wish it to be understood,however, that the specific examples presented should be regarded asillustrative only of the underlying principle of the invention which, aswill become obvious, is subject to many variations and modificationscommg within the broader scope of the invention, as expressed in theappended claims.

Figure 1 illustrates a known circuit presented for the sake ofillustration and clearer understanding of the new circuit according tothe invention.

Figure 2 shows a novel circuit according to invention.

Figure 3 represents a modification of the circuit according to Figure 2.

Figured relates to another modification of the fundamental circuit ofthe invention according to Figure 2.

Similar reference characters identify similar parts throughout thedifferent views of the drawing.

Referring to Figure 1 of the drawing, this illustrates a knownamplifying system with direct coupling, known also in the prior art ascontinuous current amplifier, since the output current of the last valvereadily follows the slowest variations of the input potential applied tothe first valve which latter may be a varying direct current obtainedfrom a photoelectric device, thermo-couple, or the like.

The essential feature of this circuit consists in a direct conductivecoupling connection of the anode of the input valve V1 to the grid ofthe output valve V2 in such a manner that any coupling element exceptresistance r is dispensed with. In order to secure proper grid bias forthe output valve for insuring efiicient operation under the mostfavorable conditions, a balanced circuit or bridgev system is provided.One branch of the bridge is comprised of the resistors n and 1'2 and theother branch is comprised of the cathode-anode path of the valve V1 onthe one hand and the resistance r on the other hand.

The grid-cathode path of the valve V2 forms one of the diagonal branchesof the bridge system, the remaining diagonal branch being formed by thepower source indicated by the plus and minus signs in the drawing. Insuch a system, the grid bias voltage of V2 will assume negative values,if the ratio of the direct current resistance of V1 to 1' becomessmaller than the ratioof n to n. For practicalreasons, it

was found necessary to operate direct current amplifiers from thealternating current house networks in view of the high anode voltagerequired, which can easily be secured by a power pack including theusual transformers, rectifiers, and filter elements.

This makes it necessary to use an input tube From this it follows thatimmediately after the switching-on of the amplifier, a positive gridbias equal to the full anode voltage, will be applied to the grid of thevalve V2, resulting in a high excess load being imposed upon the outputvalve during the starting period. As is obvious, this may injure ordestroy the valve unless special precaution is taken to preventoverloading. One means to obtain this effect consists in the provisionof a thermostatic switch closing the anode circuit only after the elapseof a few seconds. Such a switch has the disadvantage of beingindependable in operation and it would not function properly immediatelyfollowing the disconnection of the circuit.

Furthermore, it is known to provide a compensation circuit for applyinga special grid bias during the starting period. This, however,complicates the circuit arrangement and entails an additional voltagedrop, thus necessitating a further increase of the anode voltage and thecost of the apparatus. Also, the provision of a special auxiliaryrectifying tube for producing starting grid bias has similar drawbacks.

As a further means for decreasing excess load of the output valveduringthe starting period, it has been suggested to increase the innerresistance of the battery eliminator supplying the amplifiersufficiently so that the excess load is kept .within moderate limits. Itis obvious that this method does not constitute a favorable solution ofthe problem.

In Figure 2, we have illustrated a novel circuit in accordance with theinvention for overcoming the aforementioned disadvantages which besidesassures increased efficiency of operation. According to the new circuit,the grid of the output valve V2 is directly and conductively connectedto the cathode of the input valve V1 contrary to the usual connection tothe anode, as according to the prior art illustrated in Figure l.

The drop of the signal voltage supplied by the input valve is producedby resistance r inserted in the cathode lead of V1. This circuit alsoconstitutes a bridge circuit in principle. The resistance r and thedirect current resistance of V1 constitute one branch of the bridge andthe resistances n and n, the latter being shunted by the cathode-anodepath of V2 forming the other branch of the bridge system. Thecathode-grid path of V2 again forms the one diagonal branch of thesystem.

If no anode current is flowing through V1, the grid of V2 will receive anegative bias equal to the voltage drop of T1, thus avoiding the dangerof excess load of V2 during the starting. The grid bias voltage of V2will assume its proper operating value when the anode current through V1begins to flow, provided all the circuit elements are properly designed.The resistance w shunted by a capacity 0 in the cathode lead of valve V1serves to provide the proper grid biasing potential for the valve inputV1 in a manner well known.

In a circuit just described a choke coil is substituted for theresistance r in accordance with our invention, thus greatly simplifyingboth the design and operation of the amplifier. As is known, choke coilsin contrast to transformers may be designed with sufficient impedance toenable their use for audio frequency amplification. As no furthercoupling elements in such a direct choke coupling are used, theamplifier will operate highly independent of frequency.

In Figure 3 we have illustrated an amplification system utilizing achoke coil as hereinbefore described. This circuit merely diifers fromFigure 2 in that choke coil d has been substituted for resistance r. Afurther simplification is obtained by omitting-the resistance r2, inwhich case the value of resistance n has to be increased. Thus, one armof the bridge system is comprised by the cathode-anode path of V2 inplace of the combination of cathode-anode path with resistance 12connected in parallel thereto. This, as will be understood, makes itnecessary to readjust resistance 11 to restore proper balance of thesystem.

It is obvious that the resistance 12 according to Figures 2 and 4 mayalso be dispensed with, with 11 properly modified-as pointed out. It isunderstood that both a resistance 1' and choke coil d in combination maybe provided in the cathode lead of V2, thus combining Figures 2 and 3.The resistance 12 which, together with T1 forms a separate directcurrent circuit from the positive pole of the anode potential sourceover the cathode of V2 to the negative pole of the source, in some caseshas the effect of stabilizing the cathode potential of V2, thus insuringincreased balance of the circuit.

The resistance n has to be chosen in such a way that its voltage dropwill secure proper operating voltage bias for valve V2. We have alsoshown in this figure a grid bias resistor w shunted by capacity a forsecuring proper operating'bias of the grid of the input valve V1 in amanner similar as described in Figure 2. One of the main advantages ofthe choke coil coupling, as illustrated, resides in the fact of aconsiderable decrease of the anode voltage required, as compared withthe resistance coupling illustrated in Figure 2.

Figure 4 shows a circuit in accordance with the invention similar toFigure 3 for use with direct current heater type valves in which, as isknown, the valves are operated in series directly from a direct currentsupply system. In this case resistances n and T2 are connected in serieswith the cathode heater circuit as illustrated. Otherwise, the circuitis identical to the circuit shown in Figure 3.

It is understood that the circuit as described may be modified in manyways without departing from the spirit of the invention. This, forinstance, a common bulb may be used enclosing the electrode elements ofboth valves V1 and V2 with the cathode of V1 directly connected to thegrid of V2 inside the common evacuated vessel, as is known in multipletubes.

What we claim is:

1. In an electrical system, a first discharge valve; a second dischargevalve, said valves having cathode, anode, and control electrodes; asource of energy; a direct conductive coupling connection from thecathode of said first valve to the control electrode of said secondvalve; means including circuit connections for connecting the anodecircuits of said valves in parallel across said source; and a choke coilinserted in the cathode lead of said first valve.

having cathode grid and anode electrodes; a

source of anode potential supply; a direct conductive couplingconnection from the cathode of said first valve to the grid of saidsecond valve; means including circuit connections for connecting theanode of circuits of said valves in parallel across saidsource;impedance means inserted in the cathode leads of said valves comprisinga choke coil in the cathode lead of said \first valve for maintainingpredeterminedxoperating potentials of said electrodes and for producingcoupling potential for said second valve. 4.\In a system as described inclaim 3, a resistairce shunted by a bypass condenser in the cathode'lead of said first valve for producing grid bias potential for saidfirst valve.

5. In a system as described in claim 3, at least one further directcurrent circuit extending from the positive pole of said source over thecathode of said second valve, to the negative pole of said source. I

6. In an electrical system, a first thermionic valve; a secon thermionicvalve, said valves a source of anodepotential supply; means-for directlyand ,conduetively connecting the oath-X ode of said first valve to thecontrol electrode;

of said second valve; \means including circuit connection from the anodeof each of said valves to the positive" terminal of said source;

"cathodes are used.

further means including circuit connections from the cathode electrodesof said valves to the opposite terminal of said source; an inductancemeans inserted in a cathode lead of said first valve; and an impedancemeans inserted in the cathode lead of said second valve for maintainingpredetermined operating potential for said electrodes and for producingcoupling potential between said valves.

7. In a system as described in claim 6, a biasing resistor in thecathode lead of said first valve shunted by a bypass condenser forproducing grid biasing potential for said first valve.

8. In a system as described in claim 6, at least one further directcurrent circuit extending from the positive pole of said source over thecathode of said second valve to the negative pole of said source.

9. In a system as described in claim 6 in which said valves are providedwith cathodes heated indirectly from a separate heater.

10. In an electrical system; a first thermionic tube; a secondthermionic tube; said tubes having cathode, anode and grid electrodes; asource of anode potential supply; connections from the anodes of saidtubes to the positive pole of said source; a resistance and a choke coilin series connected between the cathode of said first tube and thenegative pole of said source; a further resistance connecting thecathode of said second tube with the negative pole of said source; and adirect conductive coupling connection from the cathode of said firsttube to the grid of said second tube.

11. In an electrical system as claimed in claim 10 with means forrendering the cathode potential of said tubes independent of the cathodeheating current.

12. In an electrical system as claimed in claim '10 in which the tubeswith indirectly heated WALTHER BURSTY'N. WALTER TAUSIG.

DIISCLAI Me: R

1,923,254.'++Walther Barstyvnand Walter Tausig, Berlin, Germany. DIRECTCOU15LED AMPLIFIER.

Patent dated August 22, 1933.

Disclaimer filed May 11,

1938, by the assignee, Radio Corporation of America. Hereb enters thisdisclaimer to claims 4, 6, 7, and 10 in the said specification.

[ fiiciat Gazette June 14, 1938.]

